Graphene is a material with a monatomic layer structure formed by carbon atoms that are ordered in a honeycomb crystal lattice linked by carbon-carbon covalent bonds.
Graphene is therefore a 2-dimensional crystal material with distinctive electronic properties that are different from graphite in which the stacking of many of these monoatomic graphene layers occurs.
In the case of stacks of up to 10 monoatomic graphene layers, the electronic properties of the material are interesting and distinctive enough from an industrial point of view. In particular, a clearly semimetallic behaviour is observed in stacks of less than six monoatomic graphene layers, in contrast to stacks of more layers, for which this behaviour is notably reduced, such that the electronic properties of stacks of eleven or more monoatomic graphene layers change significantly, quickly becoming closer to graphite electronic properties as the number of layers increases.
In practice, graphene for industrial applications will be used as stacks of up to 10 monoatomic graphene layers. Thus, generally speaking the use of the term graphene is allowed when the number of stacked layers is less than 11, and more specifically for stacks of less than six monoatomic graphene layers.
Graphene in its free state was first obtained in 2004 by the micromechanic cleaving of graphite using “Scotch tape”.
The method consists in separating individual graphene layers from a graphite crystal in which the graphite is formed by a great number of stacked graphene layers by using an adhesive tape.
Other methods are also known to synthesise graphene, such as chemical vapour deposition (CVD), for example. In this method graphene grows by chemical vapour deposition of hydrocarbons deposited onto a metal substrate.
These techniques described for graphene synthesis are carried out on a small scale to produce small amounts, for use in laboratories, for example.
Currently, in order to produce large amounts of graphene, for industrial proposals, for example, chemical methods are used with subsequent cleaving, using thermal methods or ultrasound.
These chemical methods for obtaining graphene, use graphite as a starting material, and usually start by oxidizing it in order to obtain graphite oxide.
The most common method for obtaining graphite oxide consists of using natural graphite as a starting material, involving a step in which oxysalts are intercalated in between strong acids which decompose in the presence of air, thus producing an intermediate solid compound called graphite oxide (GO).
Amongst these methods, we can highlight that of Brodie of 1860 (KClO3 in HNO3), Staudenmeier of 1898 (KClO3 in H2SO4) and Hummers and Offemann of 1956 (KMnO4 in H2SO4).
In graphite oxide, the graphite sheets become corrugated due to the high oxidation (epoxide groups, hydroxyl groups, carboxylic groups), resulting in an increase of the distance between their sheets, to at least double such distance, thus facilitating the subsequent cleaving.
In order to produce graphene from graphite oxide, in which the graphite oxide is formed by the overlapping of hundreds of layers of graphene oxide, variants to the cleaving method of the graphite oxide have been proposed so as to produce monatomic layers or otherwise a stacking of a reduced number of monoatomic layers. One of the most interesting cleaving methods is ultrasound cleaving in a liquid medium, which produces graphene oxide particles in suspension.
This latter method produces individual sheets that are free in suspension due to the electrostatic interactions between the oxygenated functions and the liquid medium.
The advantage of synthesising graphene from carbon nanotubes has been demonstrated in the past two years. These carbon nanotubes have fewer stacked layers of graphene than graphite, thus providing a material of greater quality.
Along these lines, Kosynkin et al., using carbon nanotubes as starting material to obtain graphene nanoribbons, disclosed a modified oxidation method based on the traditional method by Hummers and Offemann.
This method is based on the method by Hummers and Offemann and it consists in a chemical oxidation to produce graphene nanoribbons by longitudinally opening and unravelling of the layers of carbon nanotubes, wherein said carbon nanotubes could be multi-walled, two-walled or single-walled.
Patent WO201022164 illustrates this manufacturing method for graphene nanoribbons.
Graphene nanoribbons consists of a graphene ribbon with a high aspect ratio, higher than 50. These ribbons could usually reach lengths ranging from 10 nm to 105 nm. and widths from 5 nm to 104 nm.
On the other hand, these nanoribbons possess highly irregular edges at an atomic scale, such that they do not have a polygonal configuration.
The object of the current invention consists in a method for obtaining high quality graphene oxide nanoplatelets in a sufficient amount to allow their use at an industrial scale.